Photosensitive polymers with photo-crosslinkable groups have gained a considerable interest in recent years owing to a wide variety of applications in the field of photoresist, photocurable coatings, microlithography, etc.
Also in the field of organic semi-conducting materials, the interest in crosslinkable materials is increasing, notably for electroluminescent devices and the application of the active layer in a direct structured manner, avoiding then the use of shadow masks.
An approach has been proposed in U.S. Pat. 2007/0290194 in which a crosslinking process using oxetane-functionalized organic semiconductors and conductors is proposed. In this procedure the crosslinking reaction is initiated by UV irradiation in the presence of at least one added onium compound as photoinitiator.
Another approach has been proposed in WO 2008/001051A2 wherein the active layer of the organic based device (mainly OLED, PLED, organic capacities touch sensor and bi-layer photovoltaics) is a semi-conducting polymer made from a polymerisable ink formulation comprising a liquid formed from a UV reactive resin monomer (monomer), a UV reactive thinner (oligomer) and a photoinitiator: the ink is configured for polymerization by UV exposure into a (semi-conducting) polymer film.
However the disadvantage of an admixed electronically active compound such as a photoinitiator is that it cannot be removed from the film after crosslinking and therefore can act as an impurity, adversely affecting the film composition, the film morphology and then affecting the functioning of the organic device. This is even more true for organic devices such as organic bulk hetero-junction solar cells in which the nano-morphology of the active layer is very sensitive to any parameter change (during the fabrication process but also during time after fabrication) and then to the power efficiency of the cell.
Other methods towards crosslinkable semi-conducting polymers, mainly polythiophenes, have been reported in which no photoinitiator is used, but they show many drawbacks:                G. Zotti in Synthetic Metals (1999) 105:135 has polymerized by electrochemistry an acrylated monomer giving a totally insoluble polymer;        B. de Ruiter in Synthetic Metals, (1996) 79:215-218 has polymerized by oxidative polymerization an acrylated monomer but this method shows interference between the acrylate chemistry and the radical chain process leading to a very difficult control of the polymer fabrication (e.g. linear polymer chain are difficult to reach, with high polydispersity values even for very short reaction times).        Zhu et al. in Journal of Macromolecular Science (2004) 41:1467-1487 reports the synthesis of epoxy-functionalized polythiophene derivatives. A slow degradation of cells made from poly(3-hexylthiophene) and a fullerene derivative of butyric acid methyl ester was observed due to a change of phase morphology.        